FeedCenter/Application/Style.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Style TargetType="{x:Type TextBox}">
        <Setter Property="Validation.ErrorTemplate">
            <Setter.Value>
                <ControlTemplate>
                    <StackPanel Orientation="Horizontal">
                        <AdornedElementPlaceholder x:Name="placeholder" />
                        <TextBlock FontSize="14"
                                   Margin="2,0"
                                   Foreground="White"
                                   Background="Red"
                                   TextAlignment="Center"
                                   Width="16"
                                   Text="!"
                                   ToolTip="{Binding AdornedElement.(Validation.Errors)[0].ErrorContent, ElementName=placeholder}" />
                    </StackPanel>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Style.Triggers>
            <Trigger Property="Validation.HasError"
                     Value="true">
                <Setter Property="ToolTip"
                        Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}" />
                <Setter Property="Margin"
                        Value="0,0,18,0" />
            </Trigger>
        </Style.Triggers>
    </Style>
</ResourceDictionary>